Methods for forming resistive-switching metal oxides for nonvolatile memory elements

ABSTRACT

Nonvolatile memory elements are provided that have resistive switching metal oxides. The nonvolatile memory elements may be formed from resistive-switching metal oxide layers. Metal oxide layers may be formed using sputter deposition at relatively low sputtering powers, relatively low duty cycles, and relatively high sputtering gas pressures. Dopants may be incorporated into a base oxide layer at an atomic concentration that is less than the solubility limit of the dopant in the base oxide. At least one oxidation state of the metal in the base oxide is preferably different than at least one oxidation sate of the dopant. The ionic radius of the dopant and the ionic radius of the metal may be selected to be close to each other. Annealing and oxidation operations may be performed on the resistive switching metal oxides. Bistable metal oxides with relatively large resistivities and large high-state-to-low state resistivity ratios may be produced.

This application claims the benefit of provisional patent applicationNo. 60/928,548, filed May 9, 2007, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This invention relates to nonvolatile memory elements, and moreparticularly, to methods for forming resistive switching memory elementsfor nonvolatile memory elements.

Nonvolatile memory elements are used in systems in which persistentstorage is required. For example, digital cameras use nonvolatile memorycards to store images and digital music players use nonvolatile memoryto store audio data. Nonvolatile memory is also used to persistentlystore data in computer environments.

Nonvolatile memory is often formed using electrically-erasableprogrammable read only memory (EPROM) technology. This type ofnonvolatile memory contains floating gate transistors that can beselectively programmed or erased by application of suitable voltages totheir terminals.

As fabrication techniques improve, it is becoming possible to fabricatenonvolatile memory elements with increasingly small dimensions. However,as device dimensions shrink, scaling issues are posing challenges fortraditional nonvolatile memory technology. This has led to theinvestigation of alternative nonvolatile memory technologies, includingresistive switching nonvolatile memory.

Resistive switching nonvolatile memory is formed using memory elementsthat have two or more stable states with different resistances. Bistablememory has two stable states. A bistable memory element can be placed ina high resistance state or a low resistance state by application ofsuitable voltages or currents. Voltage pulses are typically used toswitch the memory element from one resistance state to the other.Nondestructive read operations can be performed to ascertain the valueof a data bit that is stored in a memory cell.

Resistive switching based on nickel oxide switching elements and othertransition metal oxide switching elements has been demonstrated.Although metal oxide (MO) films such as these exhibit bistability, theresistance of these films and/or the ratio of the high-to-low resistancestates is (are) often insufficient to be of use within a practicalnonvolatile memory device. For instance, the resistance states of themetal oxide film should preferably be significant as compared to that ofthe system (e.g., the memory device and associated circuitry) so thatany change in the resistance state change is perceptible. This makes itdifficult to integrate lower resistance metal oxide films into practicalnonvolatile memory devices. For example, in a nonvolatile memory thathas conductive lines formed of a relatively high resistance metal suchas tungsten, the resistance of the conductive lines may overwhelm theresistance of the metal oxide resistive switching element. This may makeit difficult or impossible to sense the state of the bistable metaloxide resistive switching element. Similar issues can arise fromintegration of the resistive switching memory element with currentsteering elements such as diodes and/or resistors. The resistance of theresistive switching memory element (at least in its high resistancestate) is preferably significant compared to the resistance of thecurrent steering elements so that any change in the resistance state ofthe resistive switching memory element can be detected reliably.

It would therefore be desirable to be able to form highly resistivemetal oxide films and to be able to tailor the resistances of such filmsfor nonvolatile memory elements.

SUMMARY

In accordance with the present invention, nonvolatile memory elementsand methods of fabrication are provided. The nonvolatile memory elementsmay have resistive-switching metal oxide layers. Stacked nonvolatilememory element arrangements and nonvolatile memory elements withresistive-switching metal oxides that are connected in series withcurrent steering elements such as diodes and transistors may also beprovided.

The nonvolatile memory elements may be formed from bistable metaloxides. In a typical scenario, a layer of metal oxide is depositedbetween conductive layers that serve as memory element electrodes.

Process parameters may be selected to ensure that the resistivity of themetal oxide and the ratio of the oxide's high-state to low-stateresistances are relatively high. This ensures that the resistancechanges exhibited by the metal oxide layers in the nonvolatile memoryelements will not be overwhelmed by the resistance of routing lines andother components (e.g. resistors, current steering elements, etc.) in anonvolatile memory.

With one suitable arrangement, metal oxide films are sputter depositedusing a magnetron (i.e. sputtering source) at a relatively lowsputtering power density (e.g., below 6 W/cm²). A relatively highsputtering gas pressure (e.g., above 10 mTorr) may be used during themetal oxide deposition process to increase film resistivity in anotherembodiment. DC power pulses for the magnetron may have a relatively lowduty cycle (e.g., below 30%) in the case of pulsed-DC sputtering in oneembodiment. Rapid thermal anneal and oxidation steps may be used tofurther improve film resistivity (resistance) in another embodiment.

Dopants may be incorporated into the metal oxide to increase filmresistivity in one embodiment. In one embodiment, the dopant is chosento be aliovalent wherein at least one oxidation state of the dopant isdifferent from the oxidation state of the metal in the base oxide. Theionic radius of the dopant and the ionic radius of the metal may beselected to be close to each other. Dopants may be incorporated into abase metal oxide at an atomic concentration that is less than or equalto the solubility limit of the dopant in the base oxide.

A combination of the aforementioned process parameters, processes, andpost processing can also be used to tailor (e.g. increase) the depositedmetal oxide film resistivity (i.e. resistance of the resistanceswitching memory element).

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative array of resistive switchingmemory elements in accordance with an embodiment of the presentinvention.

FIG. 2A is a cross-sectional view of an illustrative resistive switchingnonvolatile memory element in accordance with an embodiment of thepresent invention.

FIG. 2B is a cross-sectional view of an illustrative resistive switchingnonvolatile memory element in accordance with another embodiment of thepresent invention.

FIG. 3 is a graph showing how resistive switching nonvolatile memoryelements of the types shown in FIGS. 2A and 2B may exhibit bistablebehavior in accordance with an embodiment of the present invention.

FIG. 4 is a schematic diagram of an illustrative resistive switchingmemory element in series with a diode in accordance with an embodimentof the present invention.

FIG. 5 is a schematic diagram of an illustrative resistive switchingmemory element in series with an electrical device in accordance with anembodiment of the present invention.

FIG. 6 is a schematic diagram of an illustrative resistive switchingmemory element in series with two electrical devices in accordance withan embodiment of the present invention.

FIG. 7 is a flow chart of illustrative steps involved in formingresistive switching memory elements for nonvolatile memory devices inaccordance with an embodiment of the present invention.

FIG. 8 is a table showing the effect of changes in sputtering power onthe measured resistivity of a resistive switching metal oxide inaccordance with an embodiment of the present invention.

FIG. 9 is a table showing the effect of sputtering pulse duty cycle onthe measured resistivity of a resistive switching metal oxide inaccordance with an embodiment of the present invention.

FIG. 10 is a table showing the effect of performing rapid thermaloxidation operations on deposited resistive switching metal oxides inaccordance with an embodiment of the present invention.

FIG. 11 is a graph showing the effect of performing a rapid thermaloxidation on a resistive switching memory element in accordance with anembodiment of the present invention.

FIG. 12 is a table showing how doped resistive switching metal oxidesmay exhibit enhanced resistivities relative to undoped resistiveswitching metal oxides in accordance with an embodiment of the presentinvention.

FIG. 13 is a table showing the effect of substrate temperature on themeasured resistivity of deposited metal oxide films in accordance withan embodiment of the present invention.

FIGS. 14 and 15 are tables showing how increased deposition pressuresmay enhance measured film resistivities for deposited metal oxides inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention relate to nonvolatile memory formedfrom resistive switching elements. Embodiments of the invention alsorelate to fabrication methods that may be used to form nonvolatilememory having resistive switching memory elements.

Resistive switching elements may be formed on any suitable type ofintegrated circuit. Most typically, resistive switching memory elementsmay be formed as part of a high-capacity nonvolatile memory integratedcircuit. Nonvolatile memory integrated circuits are often used inportable devices such as digital cameras, mobile telephones, handheldcomputers, and music players. In some arrangements, a nonvolatile memorydevice may be built into mobile equipment such as a cellular telephone.In other arrangements, nonvolatile memory devices are packaged in memorycards or memory keys that can be removably installed in electronicequipment by a user.

The use of resistive switching memory elements to form memory arrays onmemory devices is merely illustrative. In general, any suitableintegrated circuit may be formed using the resistive switchingstructures of the present invention. Fabrication of memory arrays formedof resistive switching memory elements is described herein as anexample.

An illustrative memory array 10 of nonvolatile resistive switchingmemory elements 12 is shown in FIG. 1. Memory array 10 may be part of amemory device or other integrated circuit. Read and write circuitry isconnected to memory elements 12 using conductors 16 and orthogonalconductors 18. Conductors such as conductors 16 and conductors 18 aresometimes referred to as word lines and bit lines and are used to readand write data into the elements 12 of array 10. Individual memoryelements 12 or groups of memory elements 12 can be addressed usingappropriate sets of conductors 16 and 18. Memory elements 12 may beformed from one or more layers of materials, as indicated schematicallyby lines 14 in FIG. 1. In addition, memory arrays such as memory array10 can be stacked in a vertical fashion to make multilayer memory arraystructures.

During a read operation, the state of a memory element 12 can be sensedby applying a sensing voltage to an appropriate set of conductors 16 and18. Depending on its history, a memory element that is addressed in thisway may be in either a high resistance state or a low resistance state.The resistance of the memory element therefore determines what digitaldata is being stored by the memory element. If the memory element has ahigh resistance, for example, the memory element may be said to containa logic one (i.e., a “1” bit). If, on the other hand, the memory elementhas a low resistance, the memory element may be said to contain a logiczero (i.e., a “0” bit). During a write operation, the state of a memoryelement can be changed by application of suitable write signals to anappropriate set of conductors 16 and 18.

A cross-section of an illustrative embodiment of a resistive switchingmemory element is shown in FIG. 2A. In the example of FIG. 2A, memoryelement 12 is formed from a metal oxide 22 and has conductive electrodes20 and 24. When constructed as part of an array such as array 10 of FIG.1, conductive lines such as lines 16 and 18 may be physically andelectrically connected to electrodes 20 and 24. Such conductive linesmay be formed from any suitable metals (e.g., tungsten, aluminum,copper, metal silicides, etc.). Conductive lines 16 and 18 may also beformed from other conductive materials (e.g., doped polysilicon, dopedsilicon, etc.) or combinations of conductive materials. If desired,conductive line 16 and conductive line 18 may serve as both conductivelines and as electrodes. In this type of arrangement, line 16 may serveas electrode 20, so that no separate conductor is needed to form anupper electrode for element 12. Similarly, line 18 may serve aselectrode 24, so that no separate conductor is needed for the lowerelectrode of element 12.

In the diagram of FIG. 2A, conductive lines 16 and 18 are shownschematically as being formed in contact with electrodes 20 and 24.Other arrangements may be used if desired. For example, there may beintervening electrical components (e.g., diodes, p-i-n diodes, silicondiodes, silicon p-i-n diodes, transistors, etc.) that are formed betweenline 16 and electrode 20 or between line 18 and electrode 24.

Metal oxide layer 22 may be formed from a single layer of material orfrom multiple sublayers of material. As shown by dotted line 23, forexample, metal oxide 22 may be formed from metal oxide sublayer 22A andmetal oxide sublayer 22B. There may, in general, be any suitable numberof sublayers in metal oxide 22 (e.g., three or more sublayers, four ormore sublayers, etc.). The depiction of two sublayers in FIG. 2A ismerely illustrative.

Each sublayer in metal oxide 22 may be formed using a differentfabrication process and/or different materials. For example, sublayer22B may be formed by sputter deposition of material at one sputteringgas pressure (e.g., 10-50 mTorr), whereas sublayer 22A may be formed bysputter deposition of material at another sputtering gas pressure (e.g.,5 mTorr). Process parameters such as sputtering power, gas pressure,material composition, and temperature may be varied between sublayers.Sublayers such as sublayers 22A and 22B may have any suitablethicknesses. For example, sublayers 22A and 22B may have layerthicknesses of 10-150 angstroms, 10-250 angstroms, 5-2000 angstroms,etc.

If desired, there may be a series-connected electrical component betweenan electrode conductor and the resistive switching metal oxide. Anillustrative arrangement in which there is an intervening electricalcomponent 38 between conductor 24 and metal oxide 22 is shown in FIG.2B.

As indicated schematically by dotted lines 21, conductive materials suchas electrodes 24 and 20 may be formed from one or more layers ofmaterials. Examples of materials that may be used to form electrodes 20and 24 include metal (e.g., refractory or transition metals), metalalloys, metal nitrides (e.g., refractory metal nitrides,Ti_(1-x)Al_(x)N_(y), Ta_(1-x)Al_(x)N_(y), etc.), metal silicon nitrides(i.e., materials containing refractory metals, transition metals, orother metals, along with silicon and nitrogen), metal silicides, orother conductors.

Metal oxide 22 may be formed from a metal oxide such as a transitionmetal oxide (e.g., nickel-based oxide, cobalt-based oxide, copper-basedoxide, titanium-based oxide, zirconium-based oxide, hafnium-based oxide,vanadium-based oxide, niobium-based oxide, tantalum-based oxide, etc.)or from other metal oxides such as aluminum oxide. One or more dopantsmay be incorporated into metal oxide 22. Examples of dopants that may beincorporated into metal oxide 22 include Al, Ti, Co, Zr, V, and Nb whichare chosen based upon the base metal oxide 22 system.

Resistive switching memory element 12 exhibits a bistable resistance.When resistive switching memory element 12 is in a high resistancestate, it may be said to contain a logic one. When resistive switchingmemory element 12 is in a low resistance state, it may be said tocontain a logic zero. (If desired, high resistance can signify a logiczero and low resistance can signify a logic one.) The state of resistiveswitching memory element 12 may be sensed by application of a sensingvoltage. When it is desired to change the state of resistive switchingmemory element 12, read and write circuitry may apply suitable controlsignals to suitable lines 16 and 18.

By proper selection of the process parameters used to fabricate metaloxide 22, a resistive switching metal oxide may be formed that exhibitsa relatively large resistance. For example, metal oxide 22 in device 12may exhibit a high-state resistivity of at least one ohm-cm, at leastten ohm-cm, or 100 ohm-cm or more. The ratio of the high-stateresistance of element 12 to the low-state resistance of element 12 maybe greater than five or ten (as an example). It is generally desirableto have the ratio of the high-state resistance of element 12 to thelow-state resistance of element 12 to be greater than or equal to tenand more preferably, greater than or equal to one hundred to facilitateclearly defined memory states.

A current (I) versus voltage (V) plot for device 12 is shown in FIG. 3.Initially, device 12 may be in a high resistance state (e.g., storing alogic one). In this state, the current versus voltage characteristic ofdevice 12 is represented by solid line HRS 26. The high resistance stateof device 12 can be sensed by read and write circuitry associated withan array of devices 12. For example, read and write circuitry may applya read voltage V_(READ) to device 12 and can sense the resulting lowcurrent I_(L) that flows through device 12. When it is desired to storea logic zero in device 12, device 12 can be placed into itslow-resistance state. This may be accomplished by using read and writecircuitry to apply a voltage V_(SET) across terminals 16 and 18 ofdevice 12. Applying V_(SET) to device 12 causes device 12 to enter itslow resistance state, as indicated by dotted line 30. In this region,the structure of device 12 is changed (e.g., through the formation ofcurrent filaments through metal oxide 22 or other suitable mechanisms),so that, following removal of the voltage V_(SET), device 12 ischaracterized by low resistance curve LRS 28.

The low resistance state of device 12 can be sensed using the read andwrite circuitry. When a read voltage V_(READ) is applied to resistiveswitching memory element 12, the read and write circuitry will sense therelatively high current value I_(H), indicating that device 12 is in itslow resistance state. When it is desired to store a logic one in device12, device 12 can once again be placed in its high resistance state byapplying a voltage V_(RESET) to device 12. When the read and writecircuitry applies V_(RESET) to device 12, device 12 enters its highresistance state HRS, as indicated by dotted line 32. When the voltageV_(RESET) is removed from device 12, device 12 will once again becharacterized by high resistance line HRS 26.

The bistable resistance of resistive switching memory element 12 makesmemory element 12 suitable for storing digital data. Because no changestake place in the stored data in the absence of application of thevoltages V_(SET) and V_(RESET), memory formed from elements such aselement 12 is nonvolatile.

Any suitable read and write circuitry and array layout scheme may beused to construct a nonvolatile memory device from resistive switchingmemory elements such as element 12. For example, horizontal and verticallines 16 and 18 may be connected directly to the terminals of resistiveswitching memory elements 12. This is merely illustrative. If desired,other electrical devices may be associated with each element 12.

An example is shown in FIG. 4. As shown in FIG. 4, a diode 36 may beplaced in series with resistive switching memory element 12. Diode 36may be a Schottky diode, a p-n diode, a p-i-n diode, or any othersuitable diode.

If desired, other electrical components can be formed in series withresistive switching memory element 12. As shown in FIG. 5,series-connected electrical device 38 may be coupled to resistiveswitching memory element 12. Device 38 may be a diode, a transistor, orany other suitable electronic device. Because devices such as these canrectify or otherwise alter current flow, these devices are sometimesreferred to as rectifying elements or current steering elements. Asshown in FIG. 6, two electrical devices 38 may be placed in series witha resistive switching memory element 12. Electrical devices 38 may beformed as part of a nonvolatile memory element or may be formed asseparate devices at potentially remote locations relative to a resistiveswitching metal oxide and its associated electrodes.

Memory elements 12 may be fabricated in a single layer in array 10 ormay be fabricated in multiple layers forming a three-dimensional stack.An advantage of forming memory arrays such as memory array 10 of FIG. 1using a multilayer memory element scheme is that this type of approachallows memory element density to be maximized for a given chip size.

If desired, a resistive switching metal oxide layer may be formed aboveor below a current steering element such as a diode. Conductive lines 16and 18 may be electrically coupled to metal oxide 22 through a number oflayers of conductive material. There may, in general, be any suitablenumber of conductive layers associated with resistive switching memoryelement 12. These conductive layers may be used for functions such asadhesion promotion, seed layers for subsequent electrochemicaldeposition, diffusion barriers to prevent undesired materials fromdiffusing into adjacent structures, contact materials (e.g., metals,metal alloys, metal nitrides, etc.) for forming ohmic contacts with themetal oxide 22, contact materials (e.g., metals, metal alloys, metalnitrides, etc.) for forming Schottky contacts to the metal oxide 22,etc.

The conductive layers in element 12 may be formed from the sameconductive material or different conductive materials. For example, aconductive layer in element 12 may include two nickel layers or maycontain a nickel layer and a titanium nitride layer (as an example).Moreover, conductive layers in element 12 may be formed using the sametechniques or different techniques. As an example, a conductive layermay include a metal layer formed using physical vapor deposition (PVD)techniques (e.g., sputter deposition) and a metal layer formed usingelectrochemical deposition techniques.

The portions of the conductive layers in element 12 that are immediatelyadjacent to metal oxide 22 or are otherwise in close association withmetal oxide 22 are sometimes referred to as the electrodes of theresistive switching memory element 12.

In general, the electrodes of resistive switching memory element 12 mayeach include a single material, may each include multiple materials, mayinclude materials formed using a single technique or a series ofdifferent techniques, or may include combinations of such materials.

Certain metals are particularly appropriate for forming metal oxide 22.These metals may include, for example, the transition metals and theiralloys, and other metals such as Al. Examples of transition metals thatmay be used in forming metal oxide 22 include Co, Ni, Cu, Ti, Zr, Hf, V,Nb, Ta, Cr, Mo, and W.

With one particularly suitable arrangement, the metals for forming metaloxide 22 include nickel. The metal oxide 22 may include other elementsin addition to nickel. For example, metal oxide 22 may be formed from ametal such as nickel that has been doped with a dopant material such astitanium. In this situation, metal oxide 22 will contain nickel,titanium, and oxygen.

Other dopant materials that may be used include Al, Ti, Co, Zr, V, andNb (as examples) depending on the base metal oxide 22 system.

Any suitable conductive materials may be used for forming the electrodes20 and 24 of resistive switching memory element 12. Illustrativeconductive materials include transition metals (and their nitrides),refractory metals (and their nitrides, including Ti_(1-x)Al_(x)N_(y),Ta_(1-x)Al_(x)N_(y), etc.), and noble metals. Illustrative examples ofmetals that may be used as conductive materials include Ni, Ti, Co, Cu,Ta, W, Mo, Ir, Ru, and Pt. These are merely illustrative examples ofmaterials that may be used. Combinations of two or more of these metalsmay be used or other suitable conductive materials may be used aselectrodes 20 and 24, if desired.

The layers of material that are formed when fabricating elements 12 maybe deposited using any suitable techniques. Illustrative depositiontechniques include physical vapor deposition (e.g., sputter depositionor evaporation), chemical vapor deposition, atomic layer deposition,electrochemical deposition (e.g., electroless deposition orelectroplating), ion implantation (e.g., ion implantation followed byannealing operations), thermal oxidation, etc. When appropriate processparameters are used in forming the layers of elements 12 such as metaloxide layer 22, a bistable memory element with a suitably largeresistance and a satisfactorily large difference between its high-stateresistance and low-state resistance may be formed.

A typical fabrication process is shown in FIG. 7. At step 40, a firstconductive layer such as conductive layer 24 of FIGS. 2A and 2B may beformed. Conductive layer 24 may be formed on lower circuit layers.Conductive layer 24 may, as an example, be formed on an underlying line18 on device 10 or may be formed as part of line 18. Conductive layer 24may be deposited on lower layer structures that include a substrate(e.g., a silicon wafer), layers that form underlying layers ofnonvolatile memory elements (e.g., when the integrated circuit beingformed is a stacked memory device), conductive layers such as lines 18for routing, insulating layers for insulating conductive routing linesand nonvolatile memory elements from each other, or any other suitablelayers of material.

Layer 24 may be formed from metal, metal nitrides, metal silicides, orother suitable conductive materials. Techniques that may be used to formlayer 24 include physical vapor deposition (e.g., sputter deposition orevaporation), chemical vapor deposition, atomic layer deposition, andelectrochemical deposition (e.g., electroless deposition orelectroplating). If desired, more than one material may be used to formconductive layer 24. For example, conductive layer 24 may be formed frommultiple sublayers of different materials or may be formed from amixture of more than one element. The composition of layer 24 may alsobe altered using doping (e.g., by using ion implantation to add dopantto a metal or other material). The thickness of layer 24 may be in therange of 10-10000 angstroms (as an example). Layer 24 may serve as alower electrode for device 12.

After the conductive layer of step 40 has been formed, one or moreoptional layers may be formed at step 42. These layers may, as anexample, be used in forming electrical devices (current steeringelements) such as device 38 of FIG. 2B. During step 42, one or morelayers of semiconductor (e.g., doped and/or intrinsic silicon) may beformed and, if desired, one or more layers of conductor or othermaterials may be formed. If forming a diode such as diode 36 of FIG. 4,layers of n-type and p-type silicon may be deposited. The layers of step42 may be deposited on conductive layer 24 using any suitable technique(e.g., physical vapor deposition, chemical vapor deposition, atomiclayer deposition, or electrochemical deposition).

At step 44, metal oxide layer 22 may be deposited above the firstconductive layer. If no optional layers were formed at step 42, themetal oxide layer may be deposited directly on the first conductivelayer. If the optional layers of step 42 were deposited on the firstconductive layer, metal oxide layer 22 may be formed on the optionallayers, above the first conductive layer.

Metal oxide layer 22 may be formed of any suitable oxide. For example,metal oxide layer 22 may be formed from a transition metal such astitanium (i.e., to form titanium oxide). Dopants such as Al, Ni, Co, Zr,V, and Nb may be used in forming layer 22 (as examples). Dopants may beintroduced in any suitable concentration (e.g., a concentration of0-30%).

Metal oxide layer 22 may be deposited as a single layer of material oras multiple sublayers. In arrangements in which layer 22 is formed ofmultiple sublayers, each sublayer may be formed by a potentiallydistinct fabrication process using a potentially distinct set ofmaterials. For example, different sublayers in metal oxide layer 22 maybe formed at different deposition pressures, temperatures, and powerlevels (e.g., different sputtering powers when layer 22 is depositedusing physical vapor deposition or PVD techniques). Different sublayersin metal oxide layer 22 may also be formed from different materials. Forexample, one sublayer may include dopant(s) and another sublayer may notinclude dopant(s). If desired, the concentrations of the materials inlayer 22 (e.g., the base metal and/or the dopant(s)) may be variedcontinuously, so that one layer runs into the next without any abruptinterfaces.

Metal oxide layer 22 may be formed using any suitable fabricationtechnique. Examples of fabrication techniques that may be used informing layer 22 include physical vapor deposition, chemical vapordeposition, atomic layer deposition, electrochemical deposition,followed by ion-implantation, annealing or other heat treatments such asoxidation, and combinations of these techniques. Oxidation techniquesmay include thermal oxidation in a furnace, oxidation in a rapid-thermaloxidation (RTO) tool, and ion implantation of oxygen ions followed byannealing.

It is desirable to be able to form highly resistive metal oxide filmsand to be able to tailor the resistances of such films for nonvolatilememory elements. Layer 22 is preferably formed with a highly resistivematerial. The resistance states of metal oxide layer 22 shouldpreferably be significant as compared to that of the system (e.g. thememory device and associated circuitry) so that any change in theresistance state change is perceptible. This makes it difficult tointegrate lower resistance metal oxide films into practical nonvolatilememory devices. For example, in a nonvolatile memory that has conductivelines formed of a relatively high resistance metal such as tungsten, theresistance of the conductive lines may overwhelm the resistance of themetal oxide resistive switching element. This may make is difficult orimpossible to sense the state of the bistable metal oxide resistiveswitching element. Similar issues can arise from integration of theresistive switching memory element with current steering elements suchas diodes and/or resistors. The resistance states of the resistiveswitching memory element are preferably significant compared to theresistance of the current steering elements so that any change in theresistance state of the resistive switching memory element can bedetected reliably.

With one suitable arrangement, high resistivity may be obtained forlayer 22 by depositing metal oxide layer 22 using sputtering techniques.A suitable sputtering (i.e. PVD) tool that may be used to deposit layer22 can contain a radio-frequency (RF), direct current (DC) or pulsed-DCpower supply to power the magnetron source (i.e., sputtering or PVDsource).

It has been observed that high quality films with relatively highresistivities may be produced by sputter depositing the metal oxidelayer 22 at a sputtering power density (proportional to the appliedpower divided by the eroded target area) of less than or equal to 6W/cm², and more preferably at less than or equal to 4 W/cm². A sputtertarget may be used that is formed from the metal associated with themetal oxide (e.g., nickel), a mixture of the metal and one or moredopant metals (e.g., nickel and titanium), or other suitable substances(e.g., a metal oxide such as nickel oxide). More than one target may beused. Arrangements in which materials are sputtered from two or moretargets simultaneously are generally referred to as co-sputteringarrangements. Metal oxide layer 22 may be deposited using co-sputteringor any other suitable sputtering technique.

In one embodiment, reactive sputtering at elevated deposition pressuresis used to deposit metal oxide layer 22 with high resistivity. Duringsputter deposition operations, a gas mixture may be introduced into themagnetron that includes oxygen and at least one sputtering gas. Thesputtering gas is ionized by the magnetron. Ionized sputtering gasimpinges on the target and causes the target material to form adeposited film on the integrated circuit that is being fabricated. Theoxygen in the sputtering gas mixture oxidizes the deposited material toform the metal oxide.

The sputtering gas may be, as an example, argon, xenon, krypton, or acombination of these gases. If desired, other gases that ionize and areable to sputter material from a target may be used. For at least some ofthe deposition process, the total gas pressure in the magnetronsputtering chamber is preferably greater than or equal to 10 mTorr, andmore preferably greater than or equal to 20 mTorr (e.g. 40 mTorr ormore, etc.). Sputtering at these elevated pressure regimes has thebenefit of increasing the resistivity of the deposited metal oxide layer22. In arrangements in which multiple sublayers of metal oxide aredeposited, one sublayer may be deposited at a relatively high pressure(e.g., 20 mTorr or higher), whereas another sublayer may be deposited ata relatively lower pressure (e.g., 5 mTorr or less than 10 mTorr). Theelevated pressure sputtering can be performed using DC sputtering,pulsed-DC sputtering, RF sputtering, and/or any combinations thereof.

In one embodiment, for reactive sputtering using a pulsed-DC powersupply, higher metal oxide resistivities have been obtained by using arelatively low duty cycle for the power pulses applied to the sputteringsource (e.g. magnetron). Suitable duty cycles may be less than or equalto 30%, more preferably less than or equal to 20% (e.g. less than orequal to 10%). Note that straight DC may also be used but may lead toarcing and/or nodule formation on the target, which can lead to processinstability and/or particle formation, both deleterious to manufacturingimplementation. A duty cycle of 30% means that a reverse pulse (e.g.non-sputtering pulse, typically a low positive pulse) is applied for 30%of the time.

The resistivity of deposited metal oxide films may also be furtherincreased by thermal annealing. For example, the metal oxide films maybe treated in a rapid thermal anneal (RTA) or rapid thermal oxidation(RTO) tool. This type of tool is able to ramp the temperature of aprocessed wafer up and down rapidly. In a typical scenario, a wafer isramped up to a desired processing temperature in less than or equal tofive minutes. Total processing times are typically greater than or equalto 30 seconds and less than 30 minutes, but may be longer. As anexample, the deposited metal oxide film may be treated during step 44using a rapid thermal anneal or rapid thermal oxidation process at atemperature of at least 300° C. in an ambient that contains oxygen.Exposing the deposited film to oxygen at an elevated temperature leadsto thermal oxidation of the deposited metal oxide layer in order toincrease the high-state and/or low-state resistivities of metal oxidelayer 22. If desired, all or part of the oxidation may be performed byimplanting oxygen ions into the deposited film followed by an anneal(e.g., in a vacuum ambient, a forming gas ambient, or an inert gasambient) or by implantation of oxygen ions followed by a combined annealand thermal oxidation operation. A furnace (e.g. vacuum or non-vacuum)furnace can also be used instead of a RTA or RTO tool.

Any suitable combination of the aforementioned processing techniques maybe used to increase the resistivity of the deposited metal oxide layer22. For example, the metal oxide may be deposited by sputtering using agas mixture with a pressure of greater than or equal to 10 mTorr for atleast a portion of the metal oxide layer 22, using pulsed-DC sputteringat a duty cycle of less than or equal to 30% at a sputtering powerdensity of less than or equal to 6 W/cm².

Metal oxide films may be produced that have high-state resistivities ofat least one ohm-cm, preferably at least 10 ohm-cm, or more preferablyat least 100 ohm-cm. In addition, the ratio of the high-state resistanceto the low-state resistance can be tailored to be greater than five orten (as an example). It is generally desirable to have the ratio of thehigh-state resistance to the low-state resistance to be greater than orequal to ten and more preferably, greater than or equal to one hundredto facilitate clearly defined memory states.

Metal oxide layer 22 may be formed from a base metal oxide doped with atleast one dopant. Metal oxide layer 22 may have particularly suitablehigh-resistivity characteristics when a dopant is used that has at leastone oxidation state that is different than at least one oxidation stateassociated with the metal in the base metal oxide.

As an example, the base metal oxide may be titanium oxide and the dopantmay be nickel or aluminum. Nickel and aluminum are suitable dopantsbecause they each have at least one oxidation state that is differentthan at least one of the oxidation states associated with titanium oxidein the base oxide. Titanium in titanium oxide has associated oxidationstates of +4 and +3. Nickel has common oxidation states of +2 and +3, soat least one of the nickel oxidation states (i.e., +2) is different thanat least one of the titanium oxidation states (i.e., +4). Similarly,aluminum has an oxidation state of +3, which is different than the +4titanium oxidation state.

In addition, the dopant(s) are chosen depending on the intrinsicconductivity type (e.g. n-type or p-type) of the base metal oxide. Whenthe oxidation state of the dopant material is greater than the oxidationstate of the metal in a p-type base metal oxide (e.g. Group IVB dopantelements in metal deficient nickel oxide, Ni_(1-x)O, where 0≦x≦1), therewill be excess positive charge(s) relative to the base lattice.Negatively charged defect(s) (e.g. electrons) will be generated tomaintain charge neutrality. These negatively charged defect(s) (e.g.electrons) can offset the majority hole carriers of the p-type basemetal oxide to reduce the effective concentration of charged carriers,and hence reduce the conductivity (i.e. increase the resistivity) of themetal oxide film. Some examples of dopant elements for nickel oxideinclude Ti, Co, Zr, V, and Nb. These dopants possess at least oneoxidation state that is different and greater than at least oneoxidation state of nickel (+2 is generally most prevalent) in nickeloxide. On the other hand, when the oxidation state of the dopantmaterial is less than the oxidation state of the metal in a n-type basemetal oxide (e.g. Group 111A dopant elements in oxygen deficienttitanium oxide, TiO_(2-x), where 0≦x≦1), there will be excess negativecharge(s) relative to the base lattice. Positively charged defect(s)(e.g. holes) will be generated to maintain charge neutrality. Thesepositively charged defect(s) (e.g. holes) can offset the majorityelectron carriers of the n-type base metal oxide to reduce the effectiveconcentration of charged carriers, and hence reduce the conductivity(i.e. increase the resistivity) of the metal oxide film.

During the operations of step 44, the dopant is preferably added in aconcentration that is below the solubility limit for the dopant in thebase oxide. Typically, the dopant concentration is less than or equal toabout 10% atomic concentration, but is not so limited depending on thematerials system.

During the process of fabricating device 10, devices 12 are exposed to arange of processing temperatures. For example, device 10 may be exposedto processing temperatures of up to 700-900° C. (e.g., up to 800° C.) orother suitable maximum processing temperatures (e.g. for dopantactivation of a diode connected in series with the resistive switchingmemory element). There is a solubility limit associated with the dopantin the base metal oxide at these processing temperatures. The metaloxide layer is preferably deposited so that the dopant is incorporatedinto the metal oxide layer at a concentration below this solubilitylimit.

In another embodiment, dopants are chosen so as to have an ionic radiusthat is close to the ionic radius of the metal in the base metal oxide.For example, a dopant may be selected for use during step 44 that has anionic radius that is greater than or equal to 75% of the ionic radius ofthe metal in the base metal oxide and that is less than or equal to 150%of the ionic radius of the metal in the base metal oxide. Morepreferably, the dopant may be selected for use during step 44 that hasan ionic radius that is greater than or equal to 85% of the ionic radiusof the metal in the base metal oxide and that is less than or equal to125% of the ionic radius of the metal in the base metal oxide.

The dopants can be introduced via sputtering (including reactivesputtering) of an alloy target (e.g. wherein the target contains boththe base metal and the dopant specie(s) of interest) or viaco-sputtering, wherein more than one target source material is used, andany other combinations thereof. In addition, the dopant can beintroduced via deposition of multi-layers wherein at least one layercontains the dopant of interest, followed by an optional thermal anneal.The dopants can also be introduced via implantation. Note the doping ofthe base metal oxide can be combined with any of the aforementionedtechniques (e.g. sputtering at elevated pressures, sputtering at lowpower densities, using low duty cycles for pulsed-DC sputtering, thermaltreatment in an oxidizing environment, etc.) to further enhance the filmresistivity of the metal oxide layer 22.

After forming metal oxide layer 22 at step 44, one or more optionallayers may be formed on the metal oxide layer 22 at step 46. Theselayers may, as an example, be used to form electrical devices (currentsteering elements) such as devices 38 of FIG. 6. During step 46, one ormore layers of semiconductor (e.g., doped and/or intrinsic silicon) maybe formed and, if desired, one or more layers of conductor or othermaterials may be formed. If forming a diode, layers of n-type and p-typesilicon may be deposited. The layers deposited during step 46 may bedeposited on metal oxide layer 22 using any suitable technique (e.g.,physical vapor deposition, chemical vapor deposition, atomic layerdeposition, or electrochemical deposition).

At step 48, a second conductive layer may be formed. For example, alayer of conductive material such as conductor 20 of FIGS. 2A and 2B maybe formed. If one or more of the optional layers of step 46 have beendeposited, the second conductive layer may be deposited on the optionallayers above the metal oxide layer 22 that was deposited at step 44. Ifnone of the optional layers of step 46 have been deposited, the secondconductive layer may be deposited above the metal oxide layer 22. Thesecond conductive layer may serve as an upper electrode for device 12.

The second conductive layer may be formed from metal, metal nitrides,metal silicides, or other suitable conductive materials. Techniques thatmay be used to form the second conductive layer include physical vapordeposition (e.g., sputter deposition or evaporation), chemical vapordeposition, atomic layer deposition, and electrochemical deposition(e.g., electroless deposition or electroplating). If desired, more thanone material may be used to form the second conductive layer. Forexample, conductive layer 20 may be formed from multiple sublayers ofdifferent materials as shown in FIG. 2B or may be formed from a mixtureof more than one element. The thickness of layer 20 may be in the rangeof 10-10000 angstroms (as an example).

If desired, optional layers of the type deposited during step 46 (e.g.,current steering elements such as a diode) may be formed after step 48as well. For example, such optional layers may be formed on top of thesecond conductive layer that is formed during step 48 (e.g., in additionto or instead of forming the optional layers during step 46).

Resistive switching metal oxide films have been sputter deposited usinga pulsed DC magnetron sputtering tool. Experimental results are shown inFIGS. 8-15.

In the experiments of FIGS. 8-15, the target diameter was 3 inches, so asputtering power of 300 W corresponds to a sputtering power density ofabout 6.6 W/cm², a sputtering power of 150 W corresponds to a sputteringpower density of 3.3 W/cm², and a sputtering power of 100 W correspondsto a sputtering power density of 2.2 W/cm². The effect of changes insputtering power on the measured resistivity of a nickel oxide film areshown in FIG. 8. The films of FIG. 8 were deposited using a 5 mTorrtotal gas mixture pressure, a 37.5% oxygen concentration in a gasmixture containing oxygen and argon, and pulsed DC pulses at a 150 kHzpulse rate. The duty cycle of the sputtering pulses, which is equal tothe ratio of the off time (or low power time) of each DC sputteringpulse to its period (i.e., 2000 ns/6666 ns), was 30%. As shown in FIG.8, use of a sputtering power of 150 W results in a higher resistivityfilm than use of a sputtering power of 300 W. Reducing the sputteringpower to 100 W from 150 W further enhances the resistivity of the metaloxide film.

The effect of duty cycle on measured resistivity for deposited nickeloxide films is shown in the table of FIG. 9. The depositions of FIG. 9were carried out using a 300 W sputtering power, a 150 kHz pulsed DCsputtering signal, a 300° C. substrate temperature, and an oxygen andargon gas mixture having a total pressure of 5 mTorr and an oxygenconcentration of 47.4%. As shown in the table of FIG. 9, the resistivityof each deposited film increases as the duty cycle of the sputteringsignal is reduced. Sputtering power pulses with a 15% duty cycle exhibita higher resistivity than sputtering power pulses having a 20% dutycycle. Film resistivity can be further enhanced using duty cycles of 10%or less. A duty cycle of 0 represents a sputtering signal that is purelyDC. In other experiments, the use of higher duty cycles (e.g., dutycycles of 20%-40%) were investigated. In general, a trend towardsincreasing resistivities at lower duty cycles was measured. For example,use of a duty cycle of 30% was observed to produce a lower resistivitythan use of a 20% duty cycle and a higher resistivity than use of a 40%duty cycle.

FIG. 10 is a table showing the effect of performing rapid thermaloxidation operations on deposited resistive switching nickel oxidefilms. During oxidation, each sample in the table of FIG. 10 wassubjected to a rapid thermal oxidation lasting about 60 seconds (plus a15-30 second temperature ramp-up period and a 15-30 second temperatureramp-down period). The temperature (i.e., the maximum or plateautemperature) of the 60 second RTO process was 550° C. The sputtering gasmixture contained oxygen and argon and had a 9 liter/minute oxygen flowrate.

Each row in the table of FIG. 10 corresponds to a different set ofdeposition conditions. As shown under the column labeled “process” inthe first row of the FIG. 10 table, for example, sample number 1 wasdeposited using a sputtering power of 300 W, a pulsed DC sputteringpower signal, a 20 second sputtering duration, a 5 mTorr gas pressure ina gas mixture of oxygen and argon, a 38% oxygen concentration, and a300° C. substrate temperature.

The measured sheet resistance of each film is listed in the column ofthe FIG. 10 table that is labeled “Rs.” There are two entries in eachrow, the first of which corresponds to the measured sheet resistance ofthe metal oxide film before the rapid thermal oxidation process wasperformed and the second of which corresponds to the measured sheetresistance of the metal oxide film after the rapid thermal oxidationprocess was performed. For example, in the first row, the measured sheetresistance of the nickel oxide film is 59.4 ohms/square. Following rapidthermal oxidation, the measured sheet resistance of the nickel oxidefilm was “off limit” because the upper limit of the sheet resistancemeasurement tool was exceeded. The rapid thermal oxidation processtherefore increased the resistivity of the deposited metal oxide film.

As the table of FIG. 10 demonstrates, each of the deposited metal filmsexhibited an increase in sheet resistance when oxidized using a rapidthermal oxidation process. (The sheet resistance of sample four was toohigh to measure both before and after RTO treatment.)

The graph of FIG. 11 further illustrates the effect of performing arapid thermal oxidation on a resistive switching metal oxide film. Asshown in the graph of FIG. 11, the metal oxide film switches between alow resistance state (LRS) having an associated resistance of 50-100 kΩand a high resistance state (HRS) having an associated resistance ofgreater than one MΩ.

In the example of FIG. 11, a nickel oxide film was tested. The nickeloxide film was doped with a 1.5% atomic concentration of titanium. Thefilm was deposited using a 150 kHz pulsed DC sputtering signal having a150 W sputtering power and a 30% duty cycle, a 25° C. substratetemperature, and a 2 sccm flow of oxygen in a gas mixture of oxygen andargon. Prior to rapid thermal oxidation, the nickel oxide film exhibiteda resistance of about 10 kΩ, as shown in the inset to FIG. 11. Noresistive switching was observed prior to rapid thermal oxidation. Afterperforming the rapid thermal oxidation, the resistive switching oxideexhibits bistability and a substantially increased HRS resistance (i.e.,1 MΩ).

The table of FIG. 12 shows how doped resistive switching metal oxidesmay exhibit enhanced resistivities relative to undoped resistiveswitching metal oxides. The table of FIG. 12 has two columns. The firstcolumn lists various metal oxide film compositions (e.g., nickel oxide,nickel oxide doped with 3% atomic concentration of titanium, nickeloxide doped with 3% atomic concentration of cobalt, etc.). The secondcolumn lists measured resistivities. As shown in the second column, themeasured resistivity of the doped metal oxide films are all greater thanthe measured resistivity of the undoped nickel oxide film.

It may be desirable to heat the substrate (e.g., the silicon wafer) uponwhich a metal oxide film is being deposited. Substrate heating may serveas a form of annealing that may help to reduce carrier-producing defectsand thereby increase film resistivity. In general, any suitablesubstrate temperatures may be used (e.g., greater than or equal to 100°C., greater than or equal to 300° C., greater than 500° C., etc.). Asshown in FIG. 13, use of higher substrate temperatures (e.g., 300° C.)tends to increase film resistivity.

If desired, metal oxide films for resistive switching elements may bedeposited as one or more sublayers. For example, a thin highly resistivesublayer may be added to a thicker metal oxide layer to enhance theresistivity of the metal oxide. The use of a metal oxide film thatcontains a thin highly resistive layer rather than a single layer maymake it easier to achieve a given film resistivity with a desired filmstability. The effect of forming the metal oxide layer from twosublayers has been investigated for titanium oxide (nominally TiO₂)films. A 250 angstrom single-layer titanium oxide film was depositedusing a 5 mTorr gas pressure. For comparison, a bilayer titanium oxidefilm was deposited that contained a 250 angstrom layer formed using a 5mTorr gas pressure and a 5 angstrom high-resistance layer formed using a50 mTorr gas pressure.

The resistance of these two layers was compared by measuring the meancurrent passed by each layer in its high resistance state at an appliedvoltage of 0.5 volts. The single layer of titanium oxide exhibited amean current of 411 nA, whereas the bilayer of titanium oxide exhibiteda mean current of 360 nA. Although the bilayer was 5 angstroms (2%)thicker than the single layer, the bilayer exhibited an increase inresistance of over 10%, indicating that the presence of thehigh-resistance layer was helpful in increasing resistance withoutsubstantially increasing film thickness.

The effects of different gas mixture pressures were investigated bydepositing nickel oxide films with a 150 W sputtering power, a 150 kHzpulsed DC sputtering signal, and a 300° C. substrate temperature for avariety of oxygen concentrations in a gas mixture of oxygen and argon.Experimental results are shown in FIGS. 14 and 15. In the table of FIG.14, measured metal oxide film resistivities are plotted as a function ofoxygen concentration for total gas pressures of 5 mTorr and 20 mTorr. Inthe table of FIG. 15, measured metal oxide film resistivities areplotted as a function of oxygen concentration for gas mixture pressuresof 5 mTorr and 40 mTorr. As shown in these tables, film resistivityincreased with increasing gas pressure for all values of oxygenconcentration. In other experiments, other gas pressures wereinvestigated. In general, better results (higher film resistivities)were achieved when using elevated gas pressures. For example, the filmresistivities that were measured using a 10 mTorr gas pressure werehigher than those measured using a 5 mTorr gas pressure, the filmresistivities that were measured using a 20 mTorr gas pressure werehigher than those measured using a 10 mTorr gas pressure, and the filmresistivities that were measured using a 40 mTorr gas pressure werehigher than those measured using a 20 mTorr gas pressure.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for forming a resistive switchingnonvolatile memory element comprising: forming a first conductive layer;sputter depositing a resistive switching metal oxide layer in a gasmixture that includes oxygen and at least one sputtering gas, whereinthe gas mixture has a pressure greater than or equal to 10 mTorr; andforming a second conductive layer.
 2. The method defined in claim 1wherein the gas mixture has a pressure greater than or equal to 20mTorr.
 3. The method defined in claim 1 wherein the gas mixture has apressure greater than or equal to 40 mTorr.
 4. The method defined inclaim 1 further comprising annealing the deposited metal oxide layer ata temperature of at least 300° C.
 5. The method defined in claim 1further comprising annealing the deposited metal oxide layer at atemperature of at least 300° C. for at least thirty seconds in anambient that contains oxygen.
 6. The method defined in claim 1 furthercomprising thermally oxidizing the deposited metal oxide layer.
 7. Themethod defined in claim 1 wherein sputter depositing the metal oxidelayer comprises sputter depositing the metal oxide layer at a sputteringpower density of less than or equal to 6 W/cm².
 8. The method defined inclaim 1 wherein sputter depositing the metal oxide layer comprisessputter depositing the metal oxide layer at a sputtering power densityof less than or equal to 4 W/cm².
 9. The method defined in claim 1wherein sputter depositing the metal oxide layer comprises sputterdepositing the metal oxide layer by sputtering a target using powerpulses having a duty cycle of less than or equal 20%.
 10. The methoddefined in claim 1 wherein sputter depositing the metal oxide layercomprises sputter depositing the metal oxide layer by sputtering atarget using power pulses having a duty cycle of less than or equal to10%.
 11. The method defined in claim 1 wherein sputter depositing themetal oxide layer comprises sputter depositing at least two sublayers ofmetal oxide at different sputtering gas pressures.
 12. The methoddefined in claim 1 wherein sputter depositing the resistive switchingmetal oxide layer comprises sputter depositing a layer that contains abase metal oxide formed from a metal and that contains at least onedopant, wherein at least one oxidation state associated with the dopantis different than at least one oxidation state of the metal in the basemetal oxide.
 13. The method defined in claim 12 wherein the base metaloxide is n-type and wherein the at least one oxidation state associatedwith the dopant is less than the at least one oxidation state associatedwith the metal in the base metal oxide.
 14. The method defined in claim12 wherein the base metal oxide is p-type and wherein the at least oneoxidation state associated with the dopant is greater than the at leastone oxidation state associated with the metal in the base metal oxide.15. The method defined in claim 12 wherein the base metal oxidecomprises nickel oxide and wherein the dopant comprises a materialselected from the group consisting of: Ti, Co, Zr, V, and Nb.
 16. Themethod defined in claim 12 wherein depositing the metal oxide layercomprises sputter depositing the metal oxide layer with an atomicconcentration of the dopant of less than 10%.
 17. The method defined inclaim 12 wherein there is a solubility limit associated with the dopantin the base metal oxide and wherein depositing the metal oxide layercomprises depositing the metal oxide layer so that the dopant isincorporated into the metal oxide layer at a concentration below thesolubility limit.
 18. The method defined in claim 12 wherein there is anionic radius associated with the metal in the base metal oxide andwherein the dopant has an ionic radius that is greater than or equal to75% of the ionic radius of the metal in the base metal oxide and that isless than or equal to 150% of the ionic radius of the metal in the basemetal oxide.
 19. The method defined in claim 12 wherein depositing themetal oxide layer comprises sputter depositing the metal oxide layer ina gas mixture that includes oxygen and at least one sputtering gas andwherein the gas mixture has a pressure greater than or equal to 10mTorr.
 20. The method defined in claim 12 wherein depositing the metaloxide layer comprises sputter depositing the metal oxide layer in a gasmixture that includes oxygen and at least one sputtering gas and whereinthe gas mixture has a pressure greater than or equal 10 mTorr, themethod further comprising annealing the deposited metal oxide layer at atemperature of at least 300° C. for at least thirty seconds in an oxygenambient.
 21. The method defined in claim 12 wherein depositing the metaloxide layer comprises sputter depositing the metal oxide layer at asputtering power density of less than or equal to 6 W/cm².
 22. Themethod defined in claim 12 wherein depositing the metal oxide layercomprises sputter depositing the metal oxide layer at a sputtering powerof less than or equal to 6 W/cm² in a gas mixture that includes oxygenand at least one sputtering gas and wherein the gas mixture has apressure greater than or equal to 10 mTorr.
 23. The method defined inclaim 12 wherein depositing the metal oxide layer comprises sputterdepositing the metal oxide layer at a sputtering power of less than orequal to 4 W/cm² in a gas mixture that includes oxygen and at least onesputtering gas and wherein the gas mixture has a pressure greater thanor equal to 20 mTorr.
 24. The method defined in claim 1 wherein sputterdepositing the metal oxide layer comprises sputter depositing the metaloxide layer at a substrate temperature of at least 300° C.